A major challenge in engineering tissues is mimicking the complex cellular organization and function of the native tissues of the human body. Tissue structure and function are very highly interrelated so that cellular and macromolecular organization of the tissue often brings about mechanical and biological functionality. For example, it is the circumferential arrangement of smooth muscle fiber layers that allows for change in the caliber in the lumen of blood vessels (Fawcett, 1986); the wickerwork pattern of collagen fibers in the skin give it mechanical strength (Alberts, 1994); the polygonal phenotype and complex arrangement of hepatocytes are essential for proper liver function (Boron, 2003); and the spiraling parallel arrangements of myocytes in the ventricle eject blood (Streeter, 1979; Sommer, 1995). Without proper cellular organization, an artificial tissue does not function adequately.
Many approaches to regain, for example, cardiac function depend on making layers of bundles of contractile myocytes in vitro with the intention of surgical implantation. Seeding of cardiac cells randomly into matrices or hydrogels fails because the cells are isolated from each other promoting cellular atrophy and apoptosis. The surviving cells are few in number and not electrically connected resulting in poor force production (Langer and Vacanti, 1993). Rat neonatal cells from primary culture have been grown on surfaces adhering to peptides stamped in parallel lines but are not all electrically connected laterally (Reinecke, 1999) and have all the disadvantages of two-dimensional culture. Another 2D approach involves releasing a monolayer of cells by temperature sensitive chemistry allowing the randomly oriented myocytes to contract (Shimizu, 2003). Multiple layers were tightly stacked and implanted by ‘poly-surgery’ to generate a thicker construct. Repeat surgeries at daily intervals would not be acceptable for human application.
A more promising approach involves myocytes grown on collagen networks that are mechanically paced to form 3D electrically connected strips that have been grafted in vivo (Zimmerman, 2002). This is appealing because it recreates trabeculae ensheathed with fibroblasts and endothelium ˜100 μm in diameter. However, it is not easy to see how this approach could be scaled up for surgery. The recent exterior “chain-mail jacket” approach (Zimmerman, 2006) is fraught with practical difficulties for use in human patients. This model has an outer layer of connective tissue that might well cause fibrosis and prevent the myocytes of the graft from connecting directly with the healthy heart of the host. Without a good electrical connection the electrocardiogram would show incomplete synchronous activity. The cell source also remains an unresolved challenge.
There are inadequate local repair mechanisms within the heart to deal with physical and free radical damage following ischemic injury or mechanical overload. This problem is exacerbated by a wave of programmed death (apoptosis) of susceptible myocytes resulting in contractile dysfunction (Cheng, 1996; Kajstura, 1996). As in many cell types, IGF-1 can inhibit apoptosis (Muta, 1993; Rodriquez, 1992; Beurke, 1995) and thus, delivery of IGF-1 to the stressed heart may prevent myocyte cell death and improve cardiac function. However, data from animal studies have been equivocal, and data from human clinical trials have not demonstrated any significant effects of either administration of the mature IGF-1 or growth hormone (Deurr, 1996; Fazio, 1996). This result may not be too surprising since there is no evidence showing increased secretion of growth hormone or elevation of systemic IGF-I levels following myocardial infarcts. Furthermore, there are limitations to this approach due to the bioavailability in the heart because IGF-1 binds very rapidly to proteins in the systemic circulation. However, over-expression of the IGF-1 gene in the heart has proven beneficial in eliciting cardiac hyperplasia (Reiss, 1996), inhibiting apoptosis (Li, 1997) and preventing dilation in a transgenic mouse model of cardiomyopathy (Welch, 2002). Consequently, these salutary effects were attributed to the IGF-1 produced by the muscle even though the cDNAs used could not be spliced to produce muscle specific isoforms.
Thus there exists a need in the art to develop materials and methods for improving tissue regeneration in not only cardiac tissue but other tissue as well.